[flang] Fix an assert when RESHAPE() is called on empty strings

Summary:
When a constant array of empty strings goes through contant folding, the result
is something that contains no bytes.  If this array is passed to the intrinsic
function `RESHAPE()`, we were not handling things correctly.  I fixed this by
checking for an empty destination when calling the function `CopyFrom()` on an
array of strings.

I also added a test with a couple of different examples that trigger the
problem.

Reviewers: klausler, tskeith, DavidTruby

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D84352